Moisture problems in walls can be attributed to three principal mechanisms: bulk water intrusion, water vapor diffusion, and air leakage. In recent years, control of air movement has become recognized as a major factor in determining building performance. Depending on the local climate, air leakage through walls can result in excessive efflorescence, spalling of masonry, frozen pipes, condensation and ice buildup in cavities, wet and dysfunctional insulation, mold growth as well as rain penetration, high energy costs and poor control of the building environment.
Air leakage is the uncontrolled movement of air through the building envelope. This movement of air into a building (infiltration) and out of a building (exfiltration) is caused by pressure differences produced by wind, stack or chimney effect and mechanical pressurization. Air leakage may follow such paths as holes or openings through the envelope, for example, cracks or joints between infill components and structural elements or through porous materials such as concrete block and porous insulation materials.
The method commonly used to mitigate such problems is the use of an air barrier, which is a material that offers a higher resistance to the passage of air than most other materials. Preferably, such barriers have some degree of water vapor permeance. Various types of vapor and air barriers exist for such a use. Examples of air barriers having little vapor permeance include polyethylene film, aluminum foil, some paint products, some insulation mastic adhesives and some mastic coatings have been used as vapor barriers.
Commonly known as “house wraps or sheet goods”, some preformed sheet membrane products offer a partial solution to providing a water vapor permeable or breathable air barrier system such as Tyvek spunbonded polyolefin sheet as manufactured and sold by DuPont. This type of product presents a number of difficulties in achieving an air barrier system in that multiple accessory products are required in order to create a continuous plane of air tightness throughout the building envelope. These accessories include mechanical fasteners, nails or screws, with large diameter metal or plastic plates or washers to secure the membrane, seaming tape to seal the joints of the membrane and flexible flashing for all transition areas such as tie-ins to window frames. The use of a tape and multiple mechanical fasteners which puncture the membrane compromise the integrity of the system. In addition, air barrier membranes should be structural or rigid so that they will not deform under changes in air pressure. Deformation will change the volume of a wall cavity, which could result in a pumping action. The pumping action could result in movement of humid air and defeat the original purpose of the air barrier. An alternative to sheet or membrane barriers is the application of a fluid-applied air and moisture barrier to the sheathing or masonry behind the exterior cladding of a structure. Liquid-applied barriers form a continuous-seamless structural air and moisture barrier. Fluid-applied barriers are rolled or sprayed on to the sheathing/masonry, thus there are no fastener holes where water penetration may occur. Additionally, there is no potential for mislappling or tearing as with many sheet goods. As liquid-applied barriers are bonded to the structure, either the sheathing or masonry wall, they resist deformation. Furthermore, a fluid-applied barrier in combination with venting and compartmentalizing enables the pressure behind the cladding to equalize or modify with the pressure outside. This reduces the risk of rainwater penetration caused by pressure differentials. This pressure equalizing or moderating effect is only possible when the air barrier is structural, as is the case with a fluid applied barrier.
Cementitious compounds such as portland cement stucco, which are made up almost exclusively of sand and cement, can be porous and prone to moisture penetration. The rigid nature of stucco also makes it prone to cracking which can increase the exposure to bulk water penetration or leaking. The porous nature of stucco facades can allow moisture to damage the underlying substrate if a barrier is not used. Moisture barriers are typically employed between a slip-sheet that is adjacent to the metal lath holding the stucco façade and the underlying substrate. Slip sheets are required when there is movement between the cementitious façade and the underlying substrate. Differing expansions and movements between the two can cause cracking in the cementitious façade. A slip sheet allows the façade to essentially expand and contract independently of the substrate. The addition of the slip sheet in cementitious facades adds an additional cost that is not occurred in other siding applications.
Thus, it would be advantageous to have an air and moisture barrier having the advantages of a liquid-applied barrier without need to deploy a slip sheet in the use of cementitious facades.